One way to reduce transportation costs of moving low-energy density biomass to processing plants is to pyrolyze the biomass close to the biomass production site and then transport high-energy density pyrolysis oil produced in the pyrolysis to the processing plants. During the biomass pyrolysis process, biomass particulates are conventionally mixed with hot sand carrying heat into a fluidized bed reactor. The biomass is pyrolyzed which involves heating the biomass at about 500° C. in an inert gas with the biomass being converted into pyrolysis oil vapors, char and other gases. The pyrolysis oil vapors, after separating from char and sand in a cyclone, are condensed at lower temperatures to produce liquid pyrolysis oil. The exact composition of the pyrolysis oil is highly dependent upon the biomass feedstock and operating conditions of the pyrolysis reactor. The separated sand and char are returned to a combustor where the char is combusted in air to supply the heat required for the pyrolysis reaction. The hot sand is returned to the fluidized bed reactor for further use in pyrolysis. The char combustion produces a flue gas stream containing carbon dioxide, nitrogen and water vapor. Although the carbon dioxide that is produced comes from renewable biomass, the process would be even more desirable if the carbon dioxide from the char combustion process could be captured and sequestered.
Oxyfiring or oxy-fuel combustion of fuel is a promising carbon dioxide capture process in which fuel is burned generally in the presence of high-purity oxygen, instead of air, to produce heat and a flue gas. Dry air contains roughly (by volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide and trace amounts of other gases. Normal air, as compared to dry air, contains a variable amount of water vapor, on average around 1%. The oxyfiring process avoids the need to separate nitrogen gases and other trace gases from the flue gas, which would otherwise be present if air rather than primarily oxygen were burned. Furthermore, the formation of nitrous oxide is avoided or at least substantially reduced with oxyfiring.
Chemical looping combustion (CLC) is a novel concept for capturing carbon dioxide from systems generating heat and/or power. In the CLC process, oxygen for the combustion reaction is supplied by oxidized metal oxide sorbents rather than air as in conventional combustion processes. Since the fuel is not mixed with diluent nitrogen gas, the resulting flue gas consists primarily of carbon dioxide and water vapor. The water can be readily removed through condensation and a stream of high purity carbon dioxide can be produced, which is ready for compression and sequestration, such as in a subterranean reservoir.
Mattisson et al. (Int. J. Greenhouse Gas Control, 3, 11-19, 2009) have studied metal oxides such as manganese oxide (Mn2O3), copper oxide (CuO) and cobalt oxide (Co3O4) carried on a substrate for supplying oxygen for the combustion of solid fuels. In their process, known as chemical looping combustion (CLC), fuel and oxidized metal oxide sorbents are placed in intimate contact with each other in a fuel combustion reactor. Oxygen is released from the metal oxide sorbents during combustion with the fuel thus reducing the oxidized metal oxide sorbents into reduced metal oxide sorbents. The reduced metal oxide sorbents is then captured and recharged or regenerated with oxygen from air in an appropriate oxidation or air reactor under suitable conditions. The recharged oxidized metal oxide sorbents are then returned or “looped” to the combustion reactor for combustion with more fuel.
There is a need for a process for combusting char in a pyrolysis process wherein carbon dioxide produced in the combustion is readily captured and sequestered. Furthermore, there is a need for better quality pyrolysis oil produced during pyrolysis that remains stable during its transportation and does not deteriorate. The present invention addresses these needs.